The present invention relates generally to the field of electrical power conversion, particularly to isolated resonant converters required to provide low common mode noise.
Isolated DC/DC converters often experience common mode (CM) noise that is present between the primary and secondary terminals. The common mode noise is typically generated by unbalanced currents flowing through the inter-winding capacitances of the isolation transformer generated during switching operation of the converter.
One example of a prior art converter 10 is shown in FIG. 1. The converter includes an isolation transformer T with depicted inter-winding capacitances Ci1 and Ci2 representing lumped components of the real structure. The inter-winding capacitances typically define a capacitive coupling between the primary and secondary circuits of the converter. Because of the capacitive coupling, a primary center point voltage Vm can be used to predict common mode voltage Vcm present between the primary and secondary circuits. This can be predicted by means of experimental measurement or by circuit model simulation of the circuit.
For a converter 10 according to FIG. 1, the primary center point voltage Vm voltage is graphically represented as 21 on FIG. 2, where the voltage of an input source V1 is marked for reference. As shown on FIG. 2, the center point voltage Vm features low frequency components and voltage transients with higher dv/dt. The high dv/dt voltage transients cause high capacitive currents through the inter-winding capacitances Ci1/Ci2 which are difficult to damp. Therefore the center point voltage Vm and consequently the common mode voltage Vcm tend to oscillate at high frequency.
It can be demonstrated that if the primary center point voltage Vm is stable, then noise currents flowing through the inter-winding capacitances Ci1 and Ci2 cancel each other and the common mode voltage Vcm between the primary and secondary circuits is minimized.
One solution as has previously been known in the art is to decrease the common mode voltage of a DC/DC converter 30 having a topology as represented in FIG. 3. This topology is useful in applications where the resonant choke of the converter is represented by a leakage inductance of the isolation transformer. However, it does not offer a solution for applications where a discrete resonant choke is used.
Therefore, a solution which provides low common mode voltage and includes a discrete resonant choke is required.